Demand for portable electronic devices is increasing each year. Example portable electronic devices include: laptop computers, personal data assistants (PDAs), cellular telephones, and electronic pagers. Portable electronic devices place high importance on total weight, size, and battery life for the devices. Many portable electronic devices employ rechargeable batteries such as Nickel-Cadmium (NiCad), Nickel-Metal-Hydride (NiMHi), Lithium-Ion (Li-Ion), and Lithium-Polymer based technologies.
An example charging system for a battery is shown in FIG. 3. As shown in FIG. 3, the charging system includes a power source (PS), a regulator (REG), and a battery (BATT). The power source (PS) includes a voltage source (VS) and a source resistance (RS). The regulator (102) includes a NMOS transistor (MREG), a PMOS transistor (MP1), an amplifier (AMP), a current level control circuit (CLC), and five resistors (RSNS, R1–R4).
In operation the power source provides a charging current (ICHG) to the battery through source resistance RS, PMOS transistor MP1, and resistor RNS. Resistors R1 and R2 form a voltage divider that provide a feedback signal to amplifier AMP. Amplifier AMP compares the feedback signal to a reference voltage (VREF) and provides a control signal to transistor MREG. Transistor MREG, amplifier AMP, and resistors R1 and R2 together operate as a shunt regulator that regulates the input voltage (VIN). The shunt regulator ensures safe charging of battery BATT by limiting the charging voltage (input voltage) similar to a zener diode. Resistor RSNS converts the charging current (ICHG) into a voltage (VISNS). Resistors R3 and R4 form another voltage divider that provide another feedback signal (VBSNS) for sensing the battery voltage. The current level control circuit (CLC) monitors the charging current from voltage VISNS and regulates the amount of charging current provided by transistor MP1. The current level control circuit (CLC) also monitors the sensed battery voltage (VBSNS) to determine a when to change the charging mode from constant current to constant voltage. Once the constant voltage mode is active, the current level control circuit (CLC) monitors the charging current to end the charging cycle when the sensed charging current drops below some predetermined level.